Thin-film fabrication system employing mechanical stress measurement

ABSTRACT

A system deposits a film on a substrate while determining mechanical stress experienced by the film. A substrate is provided in a deposition chamber. A support disposed in the chamber supports a circular portion of the substrate with a first surface of the substrate facing a deposition source and a second surface being reflective. An optical displacement sensor is positioned in the deposition chamber in a spaced-apart relationship with respect to a portion of the substrate&#39;s second surface located at approximately the center of the circular portion of the substrate. When the deposition source deposits a film on the first surface, a displacement of the substrate is measured using the optical displacement sensor. A processor is programmed to use the substrate displacement to determine a radius of curvature of the substrate, and to use the radius of curvature to determine mechanical stress experienced by the film during deposition.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This is a divisional application of co-pending application Ser. No.14/645,994, “MECHANICAL STRESS MEASUREMENT DURING THIN-FILMFABRICATION”, filed on Mar. 12, 2015.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to measuring mechanical stress in thin films.More specifically, the invention is a system for measuring mechanicalstress in a thin film during thin-film fabrication that includes filmdeposition and thermal annealing processes.

2. Description of the Related Art

Control of residual stress in substrate-supported thin films hashistorically presented challenges in the fabrication of thin-filmsensors, devices, and optics utilizing substrate-supported thin films.The stress can cause buckling or cracking of the film, delamination fromthe substrate, and/or substrate deformation. Measurement and control ofthin-film stress is necessary for the successful fabrication ofthin-film-based devices (e.g., semiconductors, optics, etc.) used in thefields of chemistry, mechanics, magnetics, and electricity.

Stress-induced deformation is of particular concern in the fabricationof reflective focusing and collimating X-ray optics used is a broadrange of application-specific devices to include medical imagers andspace telescopes. Briefly, the stress experienced by a device'sthin-film elements can alter the precise geometrical figure of athin-film supporting substrate thereby degrading the device's focusingor collimating properties. A thin-film reflective coating can becomposed of a single-layer metal film, or hundreds of alternatingangstrom-scale multilayered material pairs (known as X-ray multilayersin the art). For X-ray optics that utilize multilayer coatings, thenegative impact to the overall optical performance resulting fromthin-film stress-induced figure errors is two-fold in comparison tosingle-layer films. Briefly, multilayer coatings are designed so that amaximum in reflectivity occurs at a precise grazing incidence angle fora given substrate geometry and photon energy according to the Braggcondition. Therefore, stress-induced substrate deformation can cause aneffective change in the optic's designed grazing incidence angle that,in turn, can cause a significant reduction in the focusing orcollimating properties and the X-ray reflectivity.

For most thin-film deposition techniques, the stress experienced by thethin film and substrate is highly process dependent and can be minimizedto various degrees depending on material composition through theoptimization of the many process parameters. Since thin-film and/orsubstrate stresses can be reduced or eliminated through adjustment ofthin-film fabrication process, it is desirable to monitor stress in athin-film and its supporting substrate during the fabrication process.Current approaches to in-situ stress monitoring involve the use oflaser-based optical systems that direct one or more laser beams towardsthe surface of the thin film being deposited on a substrate and measurebeam deflection using precision-placed cameras/sensors. The measuredrelative displacement of the deflected beam is geometrically related tothe stress-induced substrate curvature from which the stress can becalculated using the well-known Stoney equation. Unfortunately, thereare two major drawbacks with these approaches. First, difficulties canbe encountered for deflectometry measurement techniques when attemptingto measure the stress in transparent films. Since the light is incidenton the film side, destructive interference between the film andsubstrate can occur when the optical thickness of the film approaches aquarter of a wavelength of the incident laser light. If the reflectedlight received by the detectors is not of sufficient intensity,detectors can fail to track the signal and stall. Second, to avoidcontamination of a vacuum deposition chamber required for thin-filmfabrication, laser-based monitoring equipment is mounted outside of thedeposition chamber and focused into the deposition chamber via opticalwindows built into the chamber walls. This leads to cost/size/complexityissues for in-situ stress monitoring.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod and system that facilitates in-situ measurement of film and/orsubstrate stress occurring during thin-film fabrication.

Another object of the present invention is to provide for in-situ stressmeasurement during thin-film fabrication that has minimal or no impacton the thin-film fabrication process and systems.

Other objects and advantages of the present invention will become moreobvious hereinafter in the specification and drawings.

In accordance with the present invention, a system is provided fordepositing a film on a substrate while determining mechanical stressexperienced by the film. A vacuum deposition chamber has a materialdeposition source disposed therein. A substrate is provided that has afirst surface and a second surface opposing the first surface whereinthe second surface is a specular reflective surface. A support disposedin the chamber supports a circular portion of the substrate with thefirst surface of the substrate facing the deposition source. Thecircular portion has a center. An optical displacement sensor ispositioned in the vacuum deposition chamber in a spaced-apartrelationship with respect to a portion of the substrate's second surfacelocated at approximately the center of the circular portion of thesubstrate. A processor is coupled to the optical displacement sensor.When the deposition source is adapted to deposit a film on the firstsurface, a displacement of the substrate is measured using the opticaldisplacement sensor and is provided to the processor. The processor isprogrammed to use the displacement to determine a radius of curvature ofthe substrate, and to use the radius of curvature to determinemechanical stress experienced by the film as the film is deposited onthe first surface.

BRIEF DESCRIPTION OF THE DRAWING(S)

Other objects, features and advantages of the present invention willbecome apparent upon reference to the following description of thepreferred embodiments and to the drawings, wherein correspondingreference characters indicate corresponding parts throughout the severalviews of the drawings and wherein:

FIG. 1 is a schematic view of a system for depositing a film on asubstrate while determining mechanical stress experienced by the film inaccordance with an embodiment of the present invention;

FIG. 2 is a schematic view of the system in FIG. 1 during the filmdeposition process illustrating spherical deformation of the substrateand film caused by mechanical stress during the film deposition process;

FIG. 3 is an isolated plan view of a circular substrate supported at itsperiphery by discrete supports in accordance with an embodiment of thepresent invention;

FIG. 4 is an isolated plan view of a circular substrate supported at itsperiphery by a continuous annular support in accordance with anotherembodiment of the present invention;

FIG. 5 is a schematic view of a system for depositing a film on asubstrate while simultaneously measuring temperature of the substrateand determining mechanical stress experienced by the film in accordancewith another embodiment of the present invention; and

FIG. 6 is a schematic view of a system for depositing a film on acantilevered substrate while determining mechanical stress experiencedby the film in accordance with another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to the drawings and more particularly to FIG. 1, a systemfor depositing a film on a substrate and simultaneously determiningmechanical stress in the film in accordance with an embodiment of thepresent invention is shown and is referenced generally by numeral 10. Asused herein, the terms “film” and “thin film” refer to films of one ormore layers of material(s) being deposited onto a substrate material byknown deposition processes to include, for example, physical vapordeposition, chemical vapor deposition, molecular beam epitaxy, andatomic layer deposition. While the choice of deposition system andprocess are not limitations of the present invention, the essentialfeatures of such known deposition systems will be described brieflyherein.

System 10 includes an evacuated deposition chamber 12 housing a materialdeposition source 14 (e.g., a magnetron) that, when operated, willsputter out a selected material to form a film on a substrate 16.

In general, substrate 16 can be amorphous or crystalline and of a smoothsurface quality common for thin-film deposition. Such substratematerials typically include crystalline silicon or glass. Ideally, thecrystal orientation of the substrate should allow for a high degree ofspherical symmetry under the action of uniform film stress. For example,many applications can utilize a crystalline silicon (111) or (001) waferfor substrate 16. Such silicon wafers are commercially available from avariety of sources.

Substrate 16 defines two planar and opposing surfaces 16A and 16B.Surface 16A faces deposition source 14 and surface 16A faces away fromdeposition source 14. For purposes of the present invention, surface 16Bis polished or otherwise treated to define a surface that has goodspecular reflectivity. In accordance with an embodiment of the presentinvention, substrate 16 is in a substantially horizontal orientation. Ina horizontal orientation, substrate 16 can be supported, but notconstrained, in deposition chamber 12 on (but not affixed to) one ormore supports 18 that lie on the circumference of a circle. As will beexplained further below, support(s) 18 can be realized a plurality ofdiscrete supports distributed about the circumference of a circle or bya single annular support that defines the circle of support. In eithercase, substrate 16 simply rests on support(s) 18 which can be located atthe periphery of substrate 16 (e.g., in the case of a circular substrateas is generally the case for commercially-available silicon wafers), orwithin the confines of the periphery of a substrate without departingfrom the scope of the present invention. As will be explained furtherbelow, substrate 16 can be other geometric shapes (e.g., rectangular)and/or could be fixed (e.g., clamped) to one or more supports in otherembodiments without departing from the scope of the present invention.

An optical displacement sensor such as a fiber optic displacement sensor20 is positioned in deposition chamber 12. More specifically and forpurposes of the present invention, a probe tip 20A of displacementsensor 20 is positioned in a spaced-apart relationship with a location16C on surface 16B of substrate 16. While location 16C could be anywhereon surface 16B, mechanical-stress measurement sensitivity is maximizedwhen location 16C is approximately the center of the support circledefined by support(s) 18 for reasons that will be explained furtherbelow. Displacement sensor 20 should be compatible with a vacuumenvironment. Such displacement sensors are available commercially from,for example, PhilTec Inc., Annapolis, Md.

Displacement sensor 20 transmits optical energy to surface 16B andreceives reflected optical energy from surface 16B via probe tip 20A.The transmitted and received optical energy travels between probe tip20A and an optical transceiver 22 via an optical fiber 24. The reflectedoptical energy can be provided to a processor 26 and used to determinemechanical stress in a film being deposited on surface 16A as will beexplained further below. Optical transceiver 22 can be located outsideof deposition chamber 12 so that only optical fiber 24 need transitionthe walls of deposition chamber 12. Prior to the start of the depositionprocess, displacement sensor 20 is calibrated by measuring thepre-process distance to location 16C. This calibrated distance definesthe “zero” point from which deformation of the substrate/film will bemeasured.

Referring now to FIG. 2, the process of thin-film deposition andsimultaneous determination of mechanical stress experienced by thin-filmwill be explained. As mentioned above, when deposition source 14 isoperated, material 30 sputtered from source 14 is deposited on surface16A to form a film 32 thereon. It should be noted that the relativethickness of film 32 is exaggerated for purposes of illustration. Duringthe deposition process, mechanical stresses in film 32 cause substrate16 to deform. Since the support of substrate 16 is provided by acircular geometry, substrate 16/film 32 will experience sphericaldeformation for plane stress in the film and for a substrate ofappropriate thickness, e.g., to form a concave spherical shape relativeto probe tip 20A (as shown) or a convex spherical shape relative toprobe tip 20A. As the deposition of film 32 and deformation of substrate16/film 32 is occurring, optical transceiver 22 is operated to measuredisplacement of location 16C on surface 16A via optical energytransmission/reflection at probe tip 20A. The displacement of location16C is used by processor 26 to determine mechanical stress experiencedby film 32 as will now be explained.

The stress in film 32 results in a bending moment of substrate 16causing a change of its curvature. From the measurement of thiscurvature, the plane stress in film 32 can be calculated using thewell-known Stoney equation

${\sigma\; h_{f}} = \frac{E_{s}h_{s}^{2}\kappa}{6\left( {1 - \vartheta_{s}} \right)}$which relates the stress force per unit width, σh_(f), to the substratecurvature, κ, through a proportionality constant described by the knowngeometric and mechanical properties of the substrate, namely, thesubstrate's thickness, h_(s), and biaxial modulus,

$\frac{E_{s}}{\left( {1 - \vartheta_{s}} \right)}.$The calculation of the in-situ film stress using the Stoney formalismrelies on measurement of the relative change in substrate curvatureduring film deposition or thermal annealing of the deposited film.

For the case of a uniform, isotropic film, the deformation mode of acircular substrate is given by the parameter

$A = {\sigma\; h_{f}\frac{D_{s}^{2}}{h_{s}^{3}}}$where D_(s) is the diameter of the substrate, h_(f) is the thickness ofthe film, and σ is the film stress. In particular and as is known in theart, the substrate deformation will be spherical and agree to within 90%of the Stoney equation provided the condition A≤0.2 A_(c) is satisfied(e.g., for a silicon substrate A_(c)=680 GPa). For example, a siliconsubstrate with a 25 mm diameter and a thickness of 100 microns willdeform spherically provided the force per unit width is less than 870N/m. The maximum value of film stress for which this exemplarysubstrate's deformation will remain spherical for a 500 nm thick filmwould be approximately 1.7 GPa. For measurement of larger values of filmstress, the condition (A≤0.2 A_(c)) can be satisfied by increasing thethickness of the substrate. Curvature is constant for all points on aspherical surface and can be simply determined by measurement of itssagittal, i.e., the point of greatest deflection for thecircularly-supported substrate/film or the center of the circularsupport in the present invention.

In the illustrated example, the curvature of the spherically-shapedsubstrate/film is calculated by the direct measurement of its sagittal,δ, for a substrate-support circle of known radius, r, through thegeometric relation

$\kappa = \frac{2\delta}{r^{2} + \delta^{2}}$For small sagittal displacements where δ<<r, the Stoney equation (usedto determine the mechanical stress) can be re-written in terms of δ as

${\sigma\; h_{f}} = {{- \frac{1}{3}}\frac{E_{s}}{\left( {1 - {\vartheta\; s}} \right)}\left( \frac{h_{s}}{r} \right)^{2}\delta}$

As mentioned above, circular support of a substrate in the presentinvention can be provided by a singular annular support or by discretesupports. Accordingly, FIG. 3 illustrates an isolated plan view ofsurface 16B of circular substrate 16 with three discrete supports 18located at the substrate's periphery. FIG. 4 illustrates an isolatedplan view of a circular substrate 16 with a single/continuous annularring support 18 at the substrate's periphery.

The system of the present invention can be readily adapted to monitortemperature of substrate 16 during film deposition. For example, FIG. 5illustrates a thermocouple 28 mounted on (or possibly forming) one ofdiscrete supports 18 such that it can monitor the temperature ofsubstrate 16. Thermocouple 28 could be connected to processor 26 tocollect temperature readings. By collecting substrate temperature dataduring film deposition, the present invention can aid in the explicitdetermination of temperature dependent stress related effects such asthose affecting sensor 20, the substrate's support mechanism(s), etc.

Thin-film fabrication typically involves thermal annealing processingafter a film is deposited on a substrate. Since such thermal annealingcan also induce mechanical stress in the substrate/film, the presentinvention could also be used to determine the mechanical stresses duringthe thermal annealing phase of thin-film fabrication. Accordingly, FIG.5 also illustrates a heat source 40 (e.g., a radiant heat source) indeposition chamber 12 that supplies the requisite heat for a thermalannealing process. Stress determination during this phase of thin-filmfabrication proceeds as described above for the deposition phase.

The ambient environment can introduce thermal and/or vibrational loadson supports 18 that ultimately affect displacement of substrate 16 andmeasurements made by sensor 20. To improve measurement accuracy, it maybe necessary to subtract out systematic errors caused by such thermaland/or vibrational loads. Accordingly, FIG. 5 also illustrates sensors42 positioned adjacent to or coupled to supports 18 for measuring one ormore of environmentally-induced thermal and vibrational load dataaffecting supports 18. Such measured data can be used by, for example,processor 26 to calculate systematic errors as part of the calibrationprocess of sensor 20.

As mentioned above, the present invention is not limited to circularsubstrates or circularly-supported regions thereof. For example, FIG. 6illustrates another embodiment of the present invention where substrate16 (e.g., a rectangular substrate) is fixed/clamped along an edgethereof by/at support 18 such that substrate 16 is cantilevered indeposition chamber 12 over sensor(s) 20. In this embodiment, one or moredisplacement sensors 20 are focused on and spaced apart from surface 16Bto measure deflection during deposition and/or annealing processes. Notethat one sensor 20 could be positioned near support 18 where nodeflection will occur to establish a reference measurement. Duringdeposition on surface 16A, substrate 16 will deflect/curve towardssensor(s) 20 with the greatest amount of deflection occurring at theoutboard end of substrate 16. While the mathematics for determining theradius of curvature of substrate 16 during deposition/annealing will bedifferent than that described above for spherical deformation, theunderlying principles for measuring radius of curvature are the same.The additional features described herein could also be included in thisembodiment.

The advantages of the present invention are numerous. The disclosedoptical measurement approach does not rely on reflection from thedeposited film and, therefore, avoids the drawbacks associated withprior art approaches whose optical elements are focused on the depositedfilm. The sensor's probe tip in the present invention is protected (fromfilm deposition) by the substrate. No optical windows are required inthe deposition chamber as only a single optical fiber is transitionedinto the deposition chamber. As a result, the present invention'sin-situ thin-film stress measurement approach provides a new paradigmfor thin-film fabrication adjustments that will ultimately lead tohigher quality thin-film devices to include optical devices andsemiconductors.

Although the invention has been described relative to specificembodiments thereof, there are numerous variations and modificationsthat will be readily apparent to those skilled in the art in light ofthe above teachings. For example, more than one optical displacementsensor could be used in cases where there is isotropy in the film.Furthermore, there can be relative motion between the substrate and theoptical sensor's probe tip. For example, the substrate could bestationary or moved (e.g., rotated, moved linearly, etc.) duringdeposition and/or annealing without departing from the scope of thepresent invention. It is therefore to be understood that, within thescope of the appended claims, the invention may be practiced other thanas specifically described.

The invention claimed is:
 1. A system for depositing a film on asubstrate while determining mechanical stress experienced by the film,comprising: a vacuum deposition chamber; a material deposition sourcedisposed in said chamber; a substrate having a first surface and asecond surface opposing said first surface wherein said second surfaceis a specular reflective surface; an annular support disposed in saidchamber for annularly supporting a circular portion of said substratewith said first surface of said substrate facing said deposition source,said circular portion having a center; an optical displacement sensorpositioned fully within said vacuum deposition chamber and exposed to aprocessing space of the vacuum deposition chamber in a spaced-apartrelationship with respect to a portion of said second surface located atapproximately said center of said circular portion of said substrate,said optical displacement sensor transmitting optical energy directlyonto said second surface through a central opening of the annularsupport wherein said optical energy reflected from said second surfaceis received by said optical displacement sensor; and a processor coupledto said optical displacement sensor wherein, when said deposition sourceis adapted to deposit a film on said first surface, a displacement ofsaid substrate is measured by the optical displacement sensor using saidoptical energy reflected from said second surface and is provided tosaid processor, wherein the processor is configured to determine aradius of curvature of said substrate from said displacement and todetermine a mechanical stress, experienced by said film as said film isdeposited on said first surface, from the radius of curvature.
 2. Asystem as in claim 1, wherein said substrate is a silicon wafer.
 3. Asystem as in claim 1, wherein said substrate is a circular wafer.
 4. Asystem as in claim 1, further comprising a heat source disposed in saidchamber.
 5. A system as in claim 1, further comprising a support sensorcoupled to said support for measuring at least one of thermal loads andvibrational loads experienced by said support.
 6. A system fordepositing a film on a substrate while determining mechanical stressexperienced by the film, comprising: a vacuum deposition chamber; amaterial deposition source disposed in said chamber; a substrate havinga first surface and a second surface opposing said first surface whereinsaid first surface faces said deposition source and said second surfaceis a specular reflective surface; an annular support disposed in saidchamber for annularly supporting a portion of said substrate along acircular geometry, said circular geometry having a center; an opticaldisplacement sensor positioned fully within said vacuum depositionchamber and exposed to a processing space of the vacuum depositionchamber in a spaced-apart relationship with respect to a portion of saidsecond surface located at approximately said center of said circulargeometry, said optical displacement sensor transmitting optical energydirectly onto said second surface through a central opening of theannular support wherein said optical energy reflected from said secondsurface is received by said optical displacement sensor; and a processorcoupled to said optical displacement sensor wherein, when saiddeposition source is adapted to deposit a film on said first surface, adisplacement of said substrate is measured by the optical displacementsensor using said optical energy reflected from said second surface andis provided to said processor, wherein the processor is configured todetermine a radius of curvature of said substrate from said displacementand to determine a mechanical stress, experienced by said film as saidfilm is deposited on said first surface, from the radius of curvature.7. A system as in claim 6, wherein said substrate is a silicon wafer. 8.A system as in claim 6, wherein said substrate is a circular wafer.
 9. Asystem as in claim 6, further comprising a heat source disposed in saidchamber.
 10. A system as in claim 6, further comprising a support sensorcoupled to said support for measuring at least one of thermal loads andvibrational loads experienced by said support.
 11. A system fordepositing a film on a substrate while determining mechanical stressexperienced by the film, comprising: a vacuum deposition chamber; amaterial deposition source disposed in said chamber; an annular supportdisposed in said chamber and adapted to annularly support a circularportion of a substrate, wherein the circular portion has a first surfaceand a second surface opposing the first surface, wherein the firstsurface of the substrate faces said deposition source, and wherein thecircular portion has a center; a specular reflective surface adapted tobe exposed at the second surface of the substrate; and an opticaldisplacement sensor positioned fully within said vacuum depositionchamber and exposed to a processing space of the vacuum depositionchamber in a spaced-apart relationship with respect to a portion of saidspecular reflective surface located at approximately the center of thecircular portion of the substrate, said optical displacement sensortransmitting optical energy directly onto said specular reflectivesurface through a central opening of the annular support wherein saidoptical energy reflected from said specular reflective surface isreceived by said optical displacement sensor wherein, when saiddeposition source is adapted to deposit a film on the first surface, adisplacement of the substrate is measured by the optical displacementsensor said optical energy reflected from said specular reflectivesurface, wherein said displacement is indicative of a radius ofcurvature of the substrate, and wherein said radius of curvature isindicative of mechanical stress experienced by said film as said film isdeposited on the first surface.